Problems in the application of the SSHAC probability method for assessing earthquake hazards at Swiss nuclear power plants

From 2000 to 2004 a large scale probabilistic seismic hazard analysis (PEGASOS) was created and performed as a research project, sponsored by the Swiss NPP utilities to improve the assessment methodology for seismic risk in support of the plant-specific seismic PRAs. The project followed the methodology of the SSHAC [Senior Seismic Hazard Analysis Committee (SSHAC), 1997. Recommendations for Probabilistic Seismic Hazard Analysis: Guidance on Uncertainty and Use of Experts. NU-REG/CR-6372] procedures at its most elaborate way-level 4. Before practical implementation was to occur, a detailed review was performed including validation tests and analysis of uncertainty propagation. This paper presents the main results of the review. The review revealed that current PSHA (Probabilistic Seismic Hazard Analysis) methodology as based on logic trees, in conjunction with the SSHAC procedures, potentially leads to a significant overestimation of the seismic hazard in areas with low seismic activity due to the inherent possibilities of unconstrained accumulation of uncertainties. The preliminary results of the project did not pass any of our logical geological-scientific benchmark tests used in our attempts to perform a validation process of the PEGASOS analysis method. Some of the problems encountered are of generic nature and shall be studied carefully before making the decision of whether or not the Swiss nuclear power industry will adopt the recommended use of SSHAC-procedures as a basis for the evaluation of the seismic hazard for individual nuclear power plant seismic PRA without correction.